netdev - Re: [PATCH] samples/bpf: Add xdp_sample

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Date:   Mon, 4 Jun 2018 15:12:16 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...e.dk>,
        Song Liu <liu.song.a23@...il.com>
Cc:     Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH] samples/bpf: Add xdp_sample_pkts example

On 06/04/2018 03:02 PM, Toke Høiland-Jørgensen wrote:
> Daniel Borkmann <daniel@...earbox.net> writes:
>> On 06/02/2018 06:22 AM, Daniel Borkmann wrote:
>>> On 05/31/2018 11:44 AM, Toke Høiland-Jørgensen wrote:
>>>> Song Liu <liu.song.a23@...il.com> writes:
>>>>> On Wed, May 30, 2018 at 9:45 AM, Toke Høiland-Jørgensen <toke@...e.dk> wrote:
>>>>>> This adds an example program showing how to sample packets from XDP using
>>>>>> the perf event buffer. The example userspace program just prints the
>>>>>> ethernet header for every packet sampled.
>>>>>>
>>>>>> Most of the userspace code is borrowed from other examples, most notably
>>>>>> trace_output.
>>>>>>
>>>>>> Note that the example only works when everything runs on CPU0; so
>>>>>> suitable smp_affinity needs to be set on the device. Some drivers seem
>>>>>> to reset smp_affinity when loading an XDP program, so it may be
>>>>>> necessary to change it after starting the example userspace program.
>>>>>
>>>>> Why does this only works when everything runs on CPU0? Is this
>>>>> something we can improve?
>>>>
>>>> Yeah, good question. Basically, the call from XDP to
>>>> bpf_perf_event_output() will fail with -EOPNOTSUPP. I tracked this down
>>>> to this if statement in __bpf_perf_event_output() in bpf_trace.c:
>>>>
>>>>> 	if (unlikely(event->oncpu != cpu))
>>>>> 		return -EOPNOTSUPP;
>>>>
>>>> I *think* that the way to fix this is for the userspace program to open
>>>> a perf file descriptor for each CPU in the system and poll all of them,
>>>> in which case the XDP program can pass the BPF_F_CURRENT_CPU flag to
>>>> access the right one.
>>> That is correct, you need one perf fd per cpu, and map them accordingly
>>> into the map slots when you use BPF_F_CURRENT_CPU.
>>
>> Given this is a sample that users are likely to copy from, I think it would
>> be great if you could fix this up so you can just pass in BPF_F_CURRENT_CPU
>> eventually. Thanks for working on this, Toke!
> 
> You're welcome! And yup, I was planning to. I'll need to add a new
> function to the trace helpers that can poll more than one fd; just
> haven't gotten around to it yet. :)

Ok, great, looking forward!

Cheers,
Daniel